Quantum supremacy is the experimental demonstration of a quantum computer's dominance and advantage over classical computers by performing calculations that were previously impossible at unmatched speeds. To confirm that quantum supremacy has been achieved, computer scientists must be able to show that a classical computer could never have solved the problem while also proving that the quantum computer can perform the calculation quickly.
Computer scientists hope that quantum supremacy will lead to the cracking of Shor's algorithm -- a currently impossible calculation that is the basis of most modern cryptography -- as well as advantages in drug development, weather forecasts, stock trades and material designs.
Quantum computing is consistently evolving; quantum computers have not yet reached a point where they can show their supremacy over classical computers. This is mostly due to the huge amount of quantum bits, or qubits, that are required to perform meaningful operations on quantum computers. As the amount of necessary gates and number of qubits increases, so does the error rate, and if the error rate gets too high, the quantum computer loses any advantage it had over the classical computer.
To successfully perform useful calculations -- such as determining the chemical properties of a substance -- a few million qubits would be necessary. Currently, the largest quantum computer design is Google's Bristlecone, with a 72-qubit quantum processor, which was released in March 2018.
Quantum computers vs. classical computers
The primary difference between quantum and classical computers is in the way they work. Classical computers process information as bits, with all computations performed in a binary language of 1s and 0s. The current in classical computers is either flowing through the transistor or not; there is no in between.
Conversely, quantum computers use quantum theory as the basis of their systems. Quantum theory focuses on the extraordinary interactions between particles on an invisible scale -- such as atoms, electrons and photons. Therefore, the binary states used in classical computers can no longer be applied to quantum computers.
Qubits can theoretically outperform the computation scale of binary bits by magnitudes. This is mostly due to quantum superposition -- or the ability for a subatomic particle to exist in two states at once. Superposition allows qubits to run specific computations on various possibilities simultaneously.
Trapped ions, photons and superconductors give quantum computers the ability to perform calculations at exceptionally fast speeds and take in massive amounts of data. However, the real value that quantum computers could provide is the ability to solve problems that are too complex for classical computers to address or that would take classical computers billions of years to answer. Quantum computers should be able to create a series of samples from a random quantum circuit that follow a specific, correct distribution.
While these advantages could lead to quantum supremacy, processors have not yet been built with all the capabilities. Classical computers continue to surprise computer scientists with computational power and their ability to solve certain types of problems. Until a quantum computer is built that solves a problem it has been proven a classical computer cannot solve, it continues to be possible that a better classical algorithm exists and quantum supremacy will not be achieved.
Applications of quantum supremacy
Some people believe a quantum computer that achieves quantum supremacy could be the most disruptive new technology since the Intel 4004 microprocessor was invented in 1971. Certain professions and areas of business will be significantly impacted by quantum supremacy. Examples include:
- The ability to perform more complex simulations on a larger scale will provide companies with improved efficiency, deeper insight and better forecasting, thus improving optimization processes.
- Enhanced simulations that model complex quantum systems, such as biological molecules, would be possible.
- Combining quantum computing with artificial intelligence (AI) could make AI immensely smarter than it is now.
- New customized drugs, chemicals and materials can be designed, modeled and modified to help cultivate new pharmaceutical, commercial or business products.
- The ability to factor extremely large numbers could break current, long-standing forms of encryption.
Overall, quantum supremacy could start a new market for devices that have the potential to boost AI, intricately model molecular interactions and financial systems, improve weather forecasts and crack previously impossible codes.
While most of these applications appear to provide nothing but benefits, quantum supremacy also has the ability to destabilize the math that underlies most current data encryption. Therefore, once quantum supremacy is achieved, computer scientists will have to completely reevaluate computer security and how to protect information and data. Unfortunately, this will become extremely difficult with the high speeds and large amounts of data that the quantum computers will be working with.
Examples of quantum supremacy
While the problem that first exemplifies quantum supremacy could be whatever computer scientists want, it is expected that they will use a problem known as random circuit sampling.
This problem requires a computer to correctly sample from the possible outputs of a random quantum circuit -- similar to a series of actions that can be performed on a set of qubits. Classical computers do not possess any fast algorithms to generate these samples; therefore, as the array of possible samples increases, classical computers become overwhelmed. If a quantum computer can efficiently pull samples in this instance, then it will prove quantum supremacy.
Importance of quantum supremacy
The first quantum algorithms were solved in the 1990s and, while the problems themselves were useless, the process provided the computer scientists who designed them with knowledge and insights they could use to develop more meaningful algorithms -- like Shor's algorithm -- which could potentially have large practical consequences.
Computer scientists hope that quantum supremacy will repeat this process and drive inventors to create a quantum computer that is capable of outperforming a classical computer -- even if it only solves a simple, useless problem -- because this work could be the key to building a beneficial and supreme quantum computer.
Some people also believe Moore's law is ending soon. This would inhibit AI research because the necessary smarter applications, such as fully autonomous cars, require huge amounts of processing power. Once quantum supremacy is reached, then quantum computing should be able to resolve this problem as well as revolutionize machine learning (ML).
Finally, quantum supremacy would greatly affect the field of theoretical computer science. For decades, scientists in this field have believed in the extended Church-Turing thesis, which states that classical computers can efficiently complete any problem that any other type of computer can accomplish. Quantum supremacy totally violates that assumption. Scientists would be forced to open their minds to a whole new world of computer science.
The future of quantum supremacy
The final goal for quantum computing is to create a fully functional, universal fault-tolerant gate computer. However, before this machine can be built, computer scientists need to develop:
- Refined error correction that doesn't require huge amounts of hardware
- Advanced algorithms that can support the uniquely complex problems
- Enhanced noise
- Qubits with less noise sensitivity, longer coherence times and increased reliability
- Quantum processors that possess thousands of qubits
The U.S. and China have been the most focused on investing in quantum projects along with organizations and businesses such as Google, Microsoft, IBM, Lockheed Martin and Alibaba. Google has developed a 72-qubit quantum processor -- called Bristlecone -- which they claim will achieve quantum supremacy by the end of 2019.
Once quantum supremacy is displayed, quantum computers will provide superior use for crunching large data sets, such as those used in cancer research, drug design, genetic engineering particle physics and weather forecasting. Unfortunately, due to superposition, programmers working on developing tools to code quantum computers are unable to view the paths that their data takes from input to output, making the debugging process highly complicated.
Furthermore, while quantum supremacy can be extremely beneficial to various industries, the breakthrough could also lead to rogue states or actors using quantum computers for destructive purposes.